System and method for improving DSL performance

ABSTRACT

A system and method is described for determining a condition of a communication line, such as the absence of a filter device on a digital subscriber line (DSL). A first signal characterized by a high upstream power is activated over the line, and a first set of parameters associated with the communication line is obtained. A second signal characterized by a low upstream power is activated, and a second set of parameters associated with the communication line is obtained. Comparison of first and second sets of parameters is indicative of a condition of the connection and presences or absence of a DSL filter on the communication line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of network communications andin particular, relates to a system and method for improving theperformance of a communication line such as an asymmetrical digitalsubscriber line (ADSL) by detecting the source of problems at a user endof the communication line.

2. Description of the Related Art

Various Digital Subscriber Loop (DSL) communication systems utilize thetelephone local subscriber loops to carry high speed digitaltransmission. Examples of DSL services include asymmetric DSL (ADSL),high-rate DSL (HDSL), very high-rate DSL (VDSL) and others. Thedifferent types of DSL service are generally referred to as xDSL. ThexDSL services share the telephone wires with traditional telephony,commonly referred to as plain old telephone service (POTS).

xDSL and POTS use different frequency bands for communication; POTSsignals are restricted to frequencies below 4 kHz, while xDSL signalsuse frequencies greater than 4 kHz. The frequency band plans depend onthe specific XDSL technology, e.g. ADSL-1 uses˜25-1100 kHz. At the endof the communication line, the xDSL and POTS channels are electricallyseperated by using filters. These filters come in two main forms: one isthe POTS splitter that splits (and combines) DSL and POTS signals, whilethe other is the inline filter that only allows POTS signals to passthrough. FIG. 1 shows the configurations in which the splitter andinline filter is used. In this document, we do not distinguish betweenthe two types of filter devices, and simply refer to them asmicrofilters, filters, DSL filters, or DSL microfilters.

FIG. 1 illustrates the installation configuration of the two commonlyused DSL microfilters, typically installed at the end user location forthe network related to the present invention. FIG. 1 shows an inlinefilter 101 that is often installed between an outside connection 103 anda local telephone 105. An inline filter usually comprises a low passfilter (bandwidth of about 4 kHz) for passing lower frequency POTSsignals. FIG. 1 shows a 3-port splitter with a line port 114 forcarrying both POTS as well as DSL signals. A POTS port 118 connects tothe telephone and carries POTS, and port 116 that connects to the DSLmodem carries DSL signals. A 3-port splitter generally combines ahigh-pass filter for passing DSL signals and a low-pass filter forpassing POTS signals. The splitter can be installed by the servicetechnician or self-installed, wheareas the inline filter is almostalways self-installed.

The microfilter reduces the DSL power imparted on to the telephoneelectronics, protecting the xDSL channel from the harmonics andintermodulation products generated due to the non-linearities present inmost telephones, referred to as non-linear echo in this document.Specifically, for ADSL, if upstream signals are allowed to reach thetelephone device, they can be upconverted in frequency due tonon-linearities in the telephone device and cause interference in thedownstream channel. The microfilter is critical in reducing this effect.In this document, upstream refers to the communication from the customerpremises equipment or CPE to the central office or CO, while downstreamrefers to communication from CO to CPE. The microfilter also protectsthe xDSL service from transients caused by the telephone going fromon-hook to off hoof (and vice-versa), as well as associated impedancechanges. Not using a microfilter can severely degrade the performance ofthe DSL channel.

Since microfilters are typically distributed to new DSL customers forself-installation, it is possible, even likely that a certain percentageof customers will neglect to install these filters. It is also possiblethat the microfilters are not connected in front of each and everytelephone in the home. In short, there can be many reasons whymicrofilters do not get installed as required, putting the DSL channelat risk from the POTS service.

Therefore, for DSL service providers, it is important to diagnose thiscondition in order to correct it and ensure a high quality DSL channel.A prior method for detecting missing microfilters tends to measure thenonlinear echo on a line directly. Excessive levels of nonlinear echocan be an indication of that a DSL micro-filter is missing. However,this method is not currently engineered into existing installations; itis not a part of upcoming ADSL standards; and upgrading currentinstallations to newer CPEs that can detect nonlinear echo can becostly.

Sending a repair truck to diagnose a missing micro-filter is extremelycostly and inefficient. By contrast, instructing a customer toself-install a filter is simple and cheap, provided the service providerknows which customers lack such filtering. Therefore, a real need existsfor a diagnostic that can detect the absence of a microfilter on a DSLline. Moreover, for practical reasons, the diagnostic technique shouldbe automated and should be controlled from the CO.

SUMMARY OF THE INVENTION

The present invention provides a system and method for detecting thecause of performance degradation or impairment on a communication line,such as can result from the absence of a microfilter at the customer endof an ADSL line. A first signal characterized by having a high upstreampower is activated over the communication line. A first set ofdownstream performance parameters is obtained over time resulting fromapplication of the first signal. A second signal having a low upstreampower is then activated over the communication line characterized byhaving a low upstream power. A second set of downstream performanceparameters is obtained over time resulting from the second signal. Thebehavior of downstream performance parameters over time is indicative ofthe impairment on the communication line. Comparison of the first setand the second set of performance parameters indicates the presence orabsence of a DSL filter device.

Examples of certain features of the invention have been summarized hererather broadly in order that the detailed description thereof thatfollows may be better understood and in order that the contributionsthey represent to the art may be appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present invention, references shouldbe made to the following detailed description of an exemplaryembodiment, taken in conjunction with the accompanying drawings, inwhich like elements have been given like numerals.

FIG. 1 illustrates the installation configuration of commonly used DSLfilters at the user end of the communication line;

FIG. 2 illustrates behavior of a performance parameter obtained from aDSL communication line experiencing various degrees of impairment;

FIG. 3 illustrates an exemplary network connection in one aspect of thepresent invention;

FIGS. 4 and 5 illustrate the various performance parameters obtainedover a communication line under various degrees of impairment;

FIG. 6 illustrates a flowchart of one aspect of the present invention;and

FIGS. 7 and 8 illustrate behavior of performance parameters obtainedfrom a communication line by activated signals having various powerlevels.

DETAILED DESCRIPTION OF THE INVENTION

In view of the above, the present invention through one or more of itsvarious aspects and/or embodiments is presented to provide one or moreadvantages, such as those noted below.

FIG. 2 shows measurements 200 of a parameter which is the capacity ofthe ADSL communication channel: the Maximum Attainable Bit Rate (MABR);also referred to as the Attainable Net Data Rate (ATTNDR) in ADSLstandards. It is important to note that this is not the Shannon capacityof the channel, but it is a capacity metric calculated by the ADSLmodems based on the signal to noise ratio (SNR) measured on the line: inshort the modem cannot operate at a bit-rate greater than the MABR. Thismetric is therefore sensitive to the modem implementation and the ADSLstandard it adheres to. Importantly, changes in signal loss and additivenoise are reflected in this parameter. FIG. 2 illustrates the impactthat a missing micro-filter has on the downstream ADSL channel. It showsthe MABR measured with a micro-filter connected in front of thetelephony circuit (201) and without a micro-filter connected (203), as afunction of the length of the loop the signal travels across. Thebit-rate is on the y-axis in thousands of bits per second (kbps), whilethe loop length is on the x-axis in thousands of feet (kft). FIG. 2demonstrates that for a loop-length greater than about 6 kft, themaximum bit-rate obtained without a micro-filter present is lower thanthe maximum bit-rate obtained with micro-filter present. This is becausethe nonlinear echo caused when the microfilter is absent adds to thenoise and lowers the SNR. It seems that bit-rates obtained on looplengths less than 6 kft seem generally unaffected by missing filters.That is only because the maximum bit-rate hits a ceiling determined bythe ADSL standard for loops shorter than 6 kft. The Shannon capacity, ifcalculated, will be higher for all loop lengths when a microfilter isused. Therefore the absence of a micro filter causes a considerable dropin ADSL downstream data rate.

FIG. 3 shows two embodiments of an exemplary system of networkconnectivity over which the method of the present invention can beutilized. A processor 330, and display 333, well known in the art areconnected to a DSLAM (Digital Subscriber Line Access Multiplexer) 307which includes a signal generator. The DSLAM is coupled to a DSLcommunication line 303. A DSLAM is a mechanism at a phone company'scentral location that terminates many DSL lines and provides the DSLlines with connectivity to the rest of the network. Upstream anddownstream performance parameters for each DSL line are commonly storedat the DSLAM. Downstream parameters are measured and computed by the CPEand are transmitted to the DSLAM (this is part of the ADSL standard) forstorage. Upstream parameters are measured and computed by the DSLAMitself. The DSLAM records these parameters for all DSL lines that itserves. The DSL line 303 is a physical loop. For purposes ofillustration of the present invention, the DSL line provides sufficientlength to give rise to the effects of a missing micro-filter. A typicalloop length could be 12 kft, and might have an associated noise level,such as −140 dBm/Hz of additive white gaussian noise. Output of the DSLline 303 is coupled to both a CPE 310 and to telephone 320, through acombination of bridge and filters 315. A subset of the differentconfigurations in which the bridge and filter can be connected are shownbelow in FIG. 3. It should be noted that the microfilter can be eitheran inline filter or a splitter. A bridge is not necessary when asplitter is used, but is needed when an inline filter is used. Eventhough only 3 configurations of bridge and filtering are shown, theinvention is not limited to any particular configuration, and isindependent of the particular configuration (for e.g. two bridges may beused back to back before using an inline filter; or two telephone setsmay be present; etc.). The processor monitors and records DSLperformance parameters for the DSL line and displays them on display 333when requested.

FIG. 4 shows measurements of performance parameters of a networkconnection, such as shown in FIG. 3. One possible diagnostic sequence ofthe present invention is discussed herein. Two ADSL profiles referred toherein as Profile-1 and Profile-2 (definition of profile follows) havingdifferent upstream bit-rates and transmitter power levels are separatelyactivated over the DSL line 303. An ADSL profile is associated with apredetermined list of configuration parameters programmed into the DSLAMthat the DSLAM and modem aim to achieve and maintain for the lifetime ofthe circuit. The DSLAM can hold a finite number of profiles in memory.The processor 330 can activate any stored profile on a DSL line in anautomated manner. The configuration parameters that define a profileinclude but are not limited to the minimum and maximum operating BitRate, the minimum and maximum SNR Margin, the forward error correction(FEC) Bytes, Symbols per codeword, Interleaved Delay, Trellis Coding,Overhead Framing, Bit Swapping, Power Boost, and Bit Rate AlarmThreshold. These configuration parameters are in both upstream anddownstream directions. As mentioned earlier, the DSLAM records theperformance parameters of the DSL line at training time, and during thecontinued operation of the DSL connection, sometimes referred to asShowTime. These parameters and their inter-dependence is brieflydescribed below.

Training the line is a negotiation process between the DSLAM and the CPEin which the two arrive at the highest possible bit-rate that is betweenthe minimum and maximum operating bit-rates specified in the ADSLprofile. In doing so, the modems settle on an SNR margin and transmittedpower that lies between the minimum and maximum allowed. If the minimumSNR margin cannot be achieved, the modems will cut back on the bit-rate,so as to ensure the stablility of the connection. Typically, long loopsrequire larger transmitted power to maintain a good quality signal atthe receiver. Also, to achieve a higher operating bit-rate on the sameDSL line typically requires larger transmitted power. The modems alsocalculate the maximum attainable bit-rate (MABR) from the SNR measuredat training time. The upstream MABR is calculated by the modem in theDSLAM and downstream MABR is calculated by the modem in the CPE. Aftertraining is over, the modems continue to optimize the operating point ofthe channel in face of changing line conditions (ShowTime). For thatpurpose, the modems allow the SNR margin to float between minimum andmaximum; change the transmitted power, change the bit-loading per ADSLsub-carrier, etc. For e.g., if additive noise increases, the modem mayallow the SNR margin to decrease. If the margin is too close to theminimum allowed, it may increase the transmitted power to compensate forthe additional noise, until the SNR margin returns to a healthy level.Higher MABR always indicates a better quality DSL line. A higher SNRmargin is also indicative of a healthier line, all other parametersbeing equal. If a DSL line requires larger transmitted power than itusually needs, it can be indicative of excessive signal loss or noise.

Profile-1 is characterized by a having high upstream maximum operatingbit-rate, the DSLAM programs Profile-1, which in turn causes the CPE totransmit at a higher power to accommodate the higher bit-rate. Thus,Profile-1 is referred to as a high power profile. Profile-2 ischaracterized by having a low upstream maximum operating bit-rate, thus,the DSLAM programs Profile-2, which accordingly causes the CPE totransmit at a lower power. Thus, Profile-2 is referred to as low powerprofile. In one embodiment of the invention, the upstream maximumoperating bit-rate of Profile-1 is 768 kbps, while that of Profile-2 is64 kbps. In the same embodiment of the invention, the loop used is 12kft in length. Under these conditions, the operating bit-rate affectsthe power that is transmitted by the DSLAM modem (upstream); and theupstream power level of Profile-1 is approximately +12 dbm (high) andthat of Profile-2 is approximately +1 dbm (low). The invention calls forProfile-1 to be established over the line. After training is over,performance data for the upstream and downstream channels is captured.Furthermore, the data is gathered multiple times after training is over,at intervals specified by the user. For the purposes of the presentdiscussion, the end of training (the moment at which a data link isestablished) indicates the 0-minute mark. In FIG. 4, for example, datais gathered at the 0-minutes mark and again 6 minutes later. Theinvention then calls for Profile-2 to be activated on the line. Similarto Profile-1, data is gathered at the 0-minute mark and at the 6-minutemark. The 12 dB power difference between the high and low power signalsis exemplary only. The power difference can be lower or higher dependingon the communication line length and noise on the line. For example, a 1dB power difference may be suitable for a shorter line length withlittle noise on the line. Higher power differences may be required forlonger lengths and higher noise levels.

FIG. 4 illustrates a display of performance parameters 400 for bothupstream 405, 406 and downstream 407, 409 transmission directions,obtained during a diagnostic sequence in which a filter is not presenton the communication line. FIG. 5 illustrates a display of performanceparameters 500 for both upstream 432, 433 and downstream 434, 435transmission directions, obtained during a diagnostic sequence in whicha DSL filter is present on the communication line. A first set ofperformance parameters 405, 412 is obtained for when the signalcomprising Profile-1 is activated, and a second set of performanceparameters 407, 409 is obtained for when the signal comprising Profile-2is activated. The first set of performance parameters 405, 407 is shownin the top row. The second set of performance parameters 406, 409 issimilarly displayed in the bottom row. Upstream performance parametersare recorded in the tables on the left side, and downstream performanceparameters are displayed in the tables on the right side. Standardperformance parameters for determining a line performance can beobserved, some of which include maximum attainable bit-rate (MABR), SNRnoise margin (NMR), and transmitted power level (PWR) that have beendescribed earlier in the document. The performance parameters areexemplary only. Additional or different performance parameters such asbit error rate can be used to determine performance differencesindicative of the cause of an impairment on the communication (DSL)line.

Referring now to FIG. 4 which depicts results when a DSL micro-filter isnot present, upstream power 401 for Profile-1 is +12 dbm, and upstreampower 402 for Profile-2 it is +1 dbm. When Profile-1 is activated, thedownstream MABR 410 slightly degrades over time from 1280 kbps at 0minutes to 1216 kbps at 6 minutes. Concurrently, the downstream NMR 412also degrades from 15 dB at 0 minutes to 13 dB at 6 minutes. Thedownstream power 415 remains at constant levels over the 6-minute testperiod (+10 dBm). When Profile-2 is activated, the downstream MABR 420increases from 1280 kbps at 0 minutes to 1504 kbps at 6 minutes, asignificant improvement, and the NMR 422 also increases from 16 dB at 0minutes to 24 dB at 6 minutes (also significant). In summary, downstreamMABR and NMR for Profile-2 improve significantly over time, while thesame parameters remain almost unchanged for Profile-1. The maindifference between Profile-1 and Profile-2 is the upstream data-rate andpower. When a filter is absent, as in the case in FIG. 4, differentupstream power will result in different levels of nonlinear echo andassociated impairment. The Downstream power 425 remains constant in bothcases.

Referring now to FIG. 5, (in which a DSL micro-filter is present), theupstream power for the two profiles is +12 dbm and +1 dbm (same as inFIG. 4). However, downstream performance parameters remain substantiallyunchanged. For example, the downstream MABR 431 for Profile-1 remainssubstantially unchanged from 0 minutes to 6 minutes. Downstream MABRalso remains substantially unchanged for Profile-2 430 (from 4640 kbpsat 0 minutes to 4672 kbps at 6 minutes). Thus, when a micro-filter ispresent, non-linear echo is substantially reduced, and the power levelof the upstream signal does not significantly affect the downstreamperformance parameters.

The results of FIGS. 4 and 5 are discussed below in general. If amicro-filter is present (FIG. 5), nonlinear echo is reduced, andperformance parameters associated with either profile tend to exhibit nosubstantial deviation over time. If a micro-filter is not present (FIG.4), then after a sufficiently long time (e.g., 6 min after training),the downstream performance parameters (i.e., NMR, MABR) manifestdeviation. More specifically, when a micro-filter is not present, thedownstream-MABR for Profile-1 (high upstream power) either degrades orremains substantially unchanged, while the downstream-MABR for Profile-2(low upstream power) increases with time. A processor 330 is able todetermine the absence of a micro-filter using the system and method ofthe present invention by monitoring the downstream performanceparameters associated with Profile-1 and Profile-2.

The deviation of performance parameters over time is important to note.The training process uses a certain pattern of upstream signals thatcreate equal amounts of non-linear echo regardless of the Profile thatis going to be imposed on the line. As a result, the downstreamperformance parameters measured at 0 minutes after training areunaffected by the upstream power level of the immediately appliedprofile (i.e., MABR at 0 minutes mark is same for both Profile-1 andProfile-2 when filter is missing). However, once Profile-1 or Profile-2is activated and the effects of the different upstream power levelsbetween Profile-1 and Profile-2 become apparent, the two profiles resultin different non-linear echo and therefore different MABR at the 6minute mark, if the filter is missing. Hence deviation in downstreamperformance is seen over time. The deviation can be seen even after 3minutes. However, in some instances, the deviation in performance maycontinue to increase even after 3 minutes. Therefore, the inventioncalls for a 6 minutes wait to ensure that enough time is allowed for theCPE to update its performance parameters and to transmit the updatedparameters back to the DSLAM.

FIG. 6 shows a flowchart 500 of one aspect of the present invention. InBox 501, a first signal comprising a training procedure and a highupstream power profile is activated on the communication line. A firstset of downstream performance parameters associated with the firstsignal is recorded in Box 503. Data can be recorded at intervals chosenby the user. One possible recording timetable could be at 0, 3, 6, 9,and 12 minutes after training is over. In Box 505, a second signalcomprising a training procedure and a low upstream power profile isactivated on the communication line. A second set of downstreamperformance parameters associated with the second signal is similarlyrecorded at Box 507. In Box 509, a comparison of the downstreamperformance parameters associated with the first signal and the secondsignal is made to determine the cause of the impairment of thecommunication link, i.e., the presence or absence of a micro-filter.

The system and method of the present invention provides for monitoringof a variety of performance parameters. FIG. 7 shows an alternativemethod for determining the presence or absence of a micro-filter. Adownstream SNR measured by the CPE for two profiles having differentupstream powers and is recorded at 0 and 9 minutes after training. Inthe present embodiment of the invention, the SNR is recordedindividually for each DMT tone. DMT stands for discrete multi tone, atechnology that uses individual frequency tones to carry data. Hence theparameter recorded is the SNR per tone, or SNR[n] where n represents theindex. The ADSL standard uses DMT transmission with 256 tones: n=[0 . .. 255]. Measurements displayed in FIG. 7 are obtained in the absence ofthe micro-filter on the telephone line. FIG. 7 shows the SNR[n] measuredwhen a signal comprising a high upstream power profile, such asProfile-1, is activated over the line connection. There is nosignificant difference in the SNR[n] measured at 0 minutes (600) and 9minutes (609) after training is over (SNR at the 9 minutes mark is onlyslightly lower than that at the 0 minute mark). FIG. 8 shows the SNR[n]measured for when a signal comprising a low upstream power profile, suchas Profile-2, is activated over the line connection. The SNR[n] measured9 minutes 619 after training is significantly higher than the SNR[n]immediately after training is over 610.

The results of FIGS. 7 and 8 are in line with the downstream MABRresults of FIGS. 4 and 5. The slight degradation over time of downstreamSNR[n] in FIG. 7, for the signal comprising Profile-1, corresponds tothe slight degradation observed in downstream parameters 412 and 410 inFIG. 4. The significant increase over time of downstream SNR[n] in FIG.8, for the signal comprising Profile-2, again corresponds to thesignificant increase observed in downstream parameters 420 and 422 inFIG. 4.

Although the invention has been described with reference to severalexemplary embodiments, it is understood that the words that have beenused are words of description and illustration, rather than words oflimitation. Changes may be made within the purview of the appendedclaims, as presently stated and as amended, without departing from thescope and spirit of the invention in its aspects. Although the inventionhas been described with reference to particular means, materials andembodiments, the invention is not intended to be limited to theparticulars disclosed; rather, the invention extends to all functionallyequivalent structures, methods, and uses such as are within the scope ofthe appended claims.

In accordance with various embodiments of the present invention, themethods described herein are intended for operation as software programsrunning on a computer processor. Dedicated hardware implementationsincluding, but not limited to, application specific integrated circuits,programmable logic arrays and other hardware devices can likewise beconstructed to implement the methods described herein. Furthermore,alternative software implementations including, but not limited to,distributed processing or component/object distributed processing,parallel processing, or virtual machine processing can also beconstructed to implement the methods described herein.

It should also be noted that the software implementations of the presentinvention as described herein are optionally stored on a tangiblestorage medium, such as: a magnetic medium such as a disk or tape; amagneto-optical or optical medium such as a disk; or a solid statemedium such as a memory card or other package that houses one or moreread-only (non-volatile) memories, random access memories, or otherre-writable (volatile) memories. A digital file attachment to e-mail orother self-contained information archive or set of archives isconsidered a distribution medium equivalent to a tangible storagemedium. Accordingly, the invention is considered to include a tangiblestorage medium or distribution medium, as listed herein and includingart-recognized equivalents and successor media, in which the softwareimplementations herein are stored.

Although the present specification describes components and functionsimplemented in the embodiments with reference to particular standardsand protocols, the invention is not limited to such standards andprotocols. Each of the standards for broadband communications links(e.g. ADSL, VDSL), the Internet, and other packet switched networktransmission (e.g., TCP/IP, UDP/IP, HTML, HTTP) represent examples ofthe state of the art. Such standards are periodically superseded byfaster or more efficient equivalents having essentially the samefunctions. Accordingly, replacement standards and protocols having thesame functions are considered equivalents.

1. A computerized method for detecting a cause of an impairment of acommunication link, comprising: a processor for activating a highupstream data rate signal on the communication link; recording a firstdownstream performance for the communication link resulting from thehigh upstream data rate signal; activating a low upstream data ratesignal on the communication link; recording a second downstreamperformance for the communication link resulting from the low upstreamdata rate signal; comparing the first and second downstream performancesfor the communication link to determine the cause of the impairment ofthe communication link; and indicating that a filter is missing from thecommunication line when the first downstream performance improves lessthan the second downstream performance over time.
 2. The method of claim1, wherein the cause of the impairment further comprises an absence of afiltering device on the communication link.
 3. The method of claim 1,wherein the communication link further comprises a digital subscriberline (DSL).
 4. The method of claim 1, wherein the low upstream data ratesignal has a power less than the high upstream data rate signal.
 5. Themethod of claim 1, wherein the low upstream data rate signal has a powerthat is at least 1 dB less than the high upstream data rate signal. 6.The method of claim 1, wherein the downstream performance furthercomprises at least one of a maximum attainable bit-rate, bit error rate,noise margin and transmit power.
 7. A computer readable mediumcontaining instructions that when executed by a computer perform amethod for detecting a cause of an impairment of a communication link,comprising: activating a high upstream data rate signal on thecommunication link; recording a first downstream performance for thecommunication link resulting from the high upstream data rate signal;activating a low upstream data rate signal on the communication link;recording a second downstream performance for the communication linkresulting in the low upstream data rate signal; comparing the first andsecond downstream performances for the communication link to determinethe cause of the impairment of the communication link; and indicatingthat a filter is missing from the communication line when the firstdownstream performance improves less than the second downstreamperformance over time.
 8. The medium of claim 7, wherein in the method,the cause of the impairment further comprises an absence of a filteringdevice on the communication link.
 9. The medium of claim 7, wherein inthe method the communication link further comprises a digital subscriberline (DSL).
 10. The medium of claim 7, wherein the low upstream datarate signal has a power less than the high power upstream signal. 11.The medium of claim 7, wherein the low upstream data rate signal has apower that is at least 1 dB less than the high upstream data ratesignal.
 12. The medium of claim 7, wherein in the method the downstreamperformance further comprises at least one of a maximum attainablebit-rate, bit error rate, noise margin and transmit power.
 13. A systemfor detecting a cause of an impairment of a communication link,comprising: a customer premises equipment (CPE) for communication on thecommunication link; and a processor programmed to command a DigitalSubscriber Line Access Multiplexer (DSLAM) to activate a low powerupstream signal from the CPE on the communication link and monitor afirst downstream performance for the communication link resulting fromthe low power upstream signal, the processor further programmed tocommand the DSLAM to activate a high power upstream signal on thecommunication link and monitor a second downstream performance for thecommunication link resulting from the high power upstream signal, theprocessor further programmed to determine the cause of the impairment ofthe communication link from the first and second downstreamperformances, and the processor further programmed to indicate that afilter is missing from the communication line when the first downstreamperformance improves less than the second downstream performance overtime.
 14. The system of claim 13, wherein the DSLAM programs a highupstream data rate on the communication link to activate a high powerupstream signal on the communication link.
 15. The system of claim 13,wherein the DSLAM programs a low upstream data rate on the communicationlink to activate a low power upstream signal on the communication link.16. The system of claim 13, wherein the cause of the impairment furthercomprises an absence of a filtering device on the communication link.17. The system of claim 13, wherein the communication link furthercomprises a digital subscriber line (DSL).
 18. The system of claim 13,wherein the low power upstream signal has a power less than the highpower upstream signal.
 19. The system of claim 13, wherein the low powerupstream signal has a power that is at least 1 dB less than the highpower upstream signal.
 20. The system of claim 13, wherein thedownstream performance further comprises at least one of a maximumattainable bit-rate, bit error rate, noise margin and transmit power.21. A system for detecting a cause of an impairment of a communicationlink, comprising: a customer premises equipment (CPU for communicationon the communication link; and a processor programmed to command aDigital Subscriber Line Access Multiplexer (DSLAM) to activate a lowpower upstream signal from the CPE on the communication link and monitora first downstream performance parameter for the communication linkresulting from the low power upstream signal, the processor furtherprogrammed to command the DSLAM to activate a high power upstream signalon the communication link and monitor a second downstream performanceparameter for the communication link resulting from the high powerupstream signal, the processor further programmed to determine the causeof the impairment of the communication link from the first and seconddownstream performance parameters, and the processor further programmedto compare the first and second downstream performance parametersincluding observing the first and second downstream performanceparameters over times; wherein the first and second performanceparameters are selected from the group of performance parametersconsisting of a maximum attainable bit-rate, a bit error rate, a noisemargin, and a transmit power.